The present invention relates generally to magnetic resonance imaging (MRI), and more particularly to a method and apparatus to rapidly acquire multi-slice MR perfusion images having a large dynamic range particularly useful in capturing both renal functionality and angiographic data simultaneously.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Visualization of blood flow into and out of the kidneys is indicative of renal function and perfusion. Having the capability to visualize the blood flow can be used to diagnose a broad range of diseases. In cases where compromised blood flow to the kidneys is suspected, perfusion imaging can be an important adjunct to magnetic resonance angiography (MRA) due to the uncertainties in the degree of stenosis as measured by MRA and its functional significance. Currently, most renal perfusion techniques are single slice acquisitions with relatively low temporal resolution. Such techniques require multiple breath-holds by the patient and therefore are time consuming and susceptible to blurring if the breath-holds are not held adequately. Further, although complete coverage of both kidneys is critical in ensuring visualization of local lesions, it is nearly impossible to do so with single slice techniques. That is, many single slice acquisitions may be reconstructed to form images of both kidneys, but such techniques are very time consuming and require multiple, exact-positioned patient breath-holds.
Contrast-enhanced MRI permits visualization of blood flow. However, rapid imaging is necessary to avoid motion artifacts. Further, the acquisition of multiple slices is required to visualize both kidneys, which often lie in different anatomical planes in their entirety. Therefore, a large dynamic range is necessary to see small changes in contrast uptake and to compensate for the relatively low signal-to-noise ratio caused by the preparatory saturation pulse and the rapid imaging acquisition.
Since MRI is the one modality that can provide morphologic, angiographic, and functional information in one session, it would be advantageous to acquire this information in a single scan. That is, since the degree of renal artery stenosis may not adequately reflect blood flow to certain organs, such as the kidneys, functional imaging is essential for renal evaluation.
It would therefore be desirable to have a method and apparatus that included a renal perfusion technique to allow rapid, high temporal resolution, multi-slice imaging of the kidneys that was not dependent upon ECG gating or periodic breath-holds. It would be additionally advantageous to have sufficiently large dynamic range to simultaneously include angiographic information of the renal arteries.